Thermostatic switch



Dec. 11, 1956 D, C `PERST 2,773,962

THERMOSTATIC SWITCH Filed oct. 1o, 1955 @,mq-H

United States Patent O 'i ruuntvros'rarrc SWITCH Dudley C. Perst, Chatham, N. J., assigner to Chatham Controls Corporation, Chatham, N. 3., a corporation of N ew `ersey Application ctoher 10, 1955, Serial No. 539,535

4 Claims. (Cl. Zilli-138) The invention relates to thermostatic switches such as are employed to control the operation of various types of apparatus by opening and closing an electric circuit in response to changes in ambient temperature.

Because of the small space generally available for the accommodation of a thermostatic switch within the conlines of the apparatus it serves, the trend is toward extreme miniaturization in switch structure. ln addition to miniaturization, it is desirable to hermetically seal the operating parts of the switch in a protective casing. However, when this is done, the electrical contacts of the switch, whose meeting faces become pitted and otherwise impaired in electrical emeciency due to the creation of arcs when the circuit is made and broken, are inaccessible for wiping, scraping or other restorative action. Consequently, prior inventors in this art have incorporated in the switch structure means to produce automatic wiping action between the contacts at the time of closing and separation.

The comparable prior art thermostatic switch structures, however, depend entirely upon thermal expansion and contraction of the sealing casing to effect both the making and breaking of electrical contact and the simuitaneous wiping action between contact faces. This is undesirable because the wiping action is too short in linear extent and is peformed under varying contact pressure as the electrical contacts approach and recede in the alternate closing and opening of the switch. As a result, uneven wear on the meeting faces of the contacts occurs.

With the above-enumerated disadvantages of the prior art contact-wiping means in mind, it is the primary object of the present invention to provide an improved switch structure in which contact-wiping action occurs while the contacts are in close and even bearing contact.

To be more explicit, one electrical contact is supported by the tubular switch casing and the other contact is supported by an element of the structure having a cceflicient of expansion much lower than that of the casing. Both contacts are mounted to open and close in a direction radial to the casing axis and these movements are actuated by a typical bimetallic element that cannot commence to function as quickly as the expansible and contractible casing in which the said element is coniined, because outside heat must be conducted through the casing thereto and that takes an appreciable length of time. Consequently, in a direct acting thermostatic switch, for instance, wherein the contacts are normally closed, wipincr action between the meeting faces of the contacts in the axial direction of expansion ot' the casing will occur before the heat has penetrated to the depth of the bimetallic element and the contacts have had time to separate. Instead, the contacts will still be held in tight, even surface contact by resilient means provided for that purpose. Similarly, in an inverse acting switch, wherein the contacts are normally open and are closed by increase in ambient temperature, subse- 2,773,962 Patented Dec. 11, 1956 ICC quent contraction of the casing will cause contact-wiping action before the reduction in ambient temperature has caused the bimetallic element to open the contacts again.

Another object of the invention is to provide novel spring means to press the electrical contacts tightly together in closed-contact condition and thereby maintain desirable Tiressure during wiping action.

Further objects, features Vand advantages of the invention will become apparent as the following specific description is read in connection with the accompanying drawing, in which:

Fig. l is a longitudinal sectional view of the simplified direct acting embodiment of the invention, on a greatly enlarged scale, showing the electrical contacts in normal closed condition; Fig. 2 is an end elevation; and Fig. 3 is a top plan view.

Fig. 4 is a side elevation of a modified form of the device, showing the same on a somewhat less enlarged l scale and partially broken away.

Referring now in detail to the drawing, wherein like reference characters designate corresponding parts in the several views, Figs. l to 3, inclusive, represent a simpliiied embodiment of the invention that is direct acting in principle of operation, i. e. the electrical contacts are normally in closed condition and will be caused to open a control circuit only upon the occurrence of a rise in ambient temperature.

The device includes a frame 10 that preferably is in the form of a thirnble-shaped tubular casing having one end closed by an end wall 11. In the interest of miniatu-rization, it is desirable to utilize frame, or casing, 10 as one side of the electrical control circuit. Therefore, casing .1.59 should be made of a metal possessing suitable high electrical conductivity, such as brass, which also happens to possess a high coefficient of expansion and thus is well adapted to serve its other useful purpose in connection with the contact-wiping action to be described in greater detail later herein.

The open end of casing l() is adapted to receive perforated cap l2, whose .reduced portion 13 is of such outside diameter that it will fit with a pressed-joint degree of tightness in said end of the casing after the meeting surfaces have been wiped with a coating 14 of soft solder in a molten condition. As a result, when the solder coating ld has become solidified there will be a perfect hermetic seal at this area of juncture between casing l0 and cap l2. A stem-like electrical terminal member 15 to complement casing 1li at the opposite side of the control circuit is mounted in centered penetrating relation to cap l2 of the casing in a molded block 16 of electrical insulation. By using glass as the insulating material, a glass-to-metal base is formed for casing lil to provide `a hermetic seal as well as insulation.

A bimetallic element 17, which is preferably in the conventional strip form and in substantially straight condition at room temperatures, has its base end a'ixed to the inner end of terminal member 15 by suitable electrically conductive means, such as soldering, brazing or welding. Bimetallic element 17 extends axially in casing7 l@ and is composed of two integrally united components i7 and i7 possessing widely separated ccefiicients of expansion in accordance with conventional practice, so that the said element will bend laterali' when subjected to a change in ambient temperature. Therefore, by making component 17 of invar and com- .ponent i7 of iron, element 17 will bend upward in l when heated.

There are two coacting electrical contacts 13 and 19 for the switch. These contacts are both preferably made of silver or platinum due to the well known high conduct-ivity `and other superior electrical characteristics of these particular metals. Contact 18 may be termed the movable contact because it is supported by the free end, or tip, of bimetalic element 17 and therefore is movable under the influence of changes in ambient temperature. On the other hand, contact 19 may be called the stationary contact because it is supported by an adjusting screw 20 that has screwthreaded engagement with a radial hole 21 in casing 10 and will be stationary with respect to the casing except during momentary radial adjustment by manipulation of said screw. Contacts i8 and 19 are in radial registration within casing It@ and, as shown in Fig. l, are in closed-circuit contact under ambient temperature conditions below that at which it is desired that the control circuit shall open.

Obviously, when the thermostatic switch is to be direct acting, as shown, the component 1'7" of bimetallic element 17, which has a high coefficient orf expansion, will be arranged on the side near to stationary conta-ct 'i9 and the other component 17' will be on the opposite or far side. However, it is very easy to convert the thermostatic switch into an inverse acting switch by simply reversing the components 17 and 17 18() degrees about the axis of bimetallic element 17 and then bending said element laterally until the contacts 13 and i9* are normally separated into open-circuit condition.

After proper adjustment of stationary contact 1Q has been effected, the juncture of screw 20 with casing hole 2l may be sealed hermetically by applying a drop of molten soft solder 22 over the outer end of the screw. However, even ir this seal be omitted, casing it? will still be impervious to dust and explosion-proof.

As has already been pointed out, it is not necessary to open casing during the normal life of the switch in order to wipe, scrape or otherwise clean the meeting faces of contacts 18 and 19, due to the automatic wiping action which occurs due to the wide difference in coetrlcients of expansion of casi-ng 10 (brass) and another part of the structure, which happens to be bimetallic element 17 whose expansion and contraction lengthwise of casing 10 is determined by its component of lower expansion coe'icient 17 (lnvar). The manner in which contact-wiping occurs will now be described.

Before bimetallic element 17 has become subjected to an increase in ambient temperature by conduction through the vmetallic mass of casing 1d so as to break the contact between contacts 13 and 1%, the said casing will have expanded lengthwise sufficiently to cause adequate wiping action between the meeting faces of the contacts in a direction lying in the momentary common pla-ne of said faces. Consequently, this wiping action will occur while the contact faces are in even bearing contact and while being pressed tightly together by the inherent resiliency of bimetallic element 17. Such periodic wiping of the contact faces will preserve them in perfect electrically ecient condition during the lite ot the switch.

Fig. 4 discloses a modified form of direct acting thermostatic switch which diiers from the preceding embodiment in the provision of a separate linear spring 23 to support the movable contact, which in this instance is designated by the numeral 24. Spring 23 tends at all times to press movable contact 2li into close bearing contact with stationary contact 19 and is made of a metal, such as stainless steel, having a coefficient of expansion lower than that of casing 1t). Therefore, it is necessary to reverse the positions of the respective cornponents of bimetallic element 17. In this instance, the component 17 having the lower coetlicient of expansion (Invar) will be on the bottom and the more expansible component 17 (iron) will be on top, so that downward curling of said element due to an increase in ambient temperature will cause its free end to bear down on spring 23 and thereby move contact 24 away from stationary Contact 19. For more eicient bearing contact between bimetallic element 17 and spring 23, the free end portion of said element is downwardly indented to provide a smoothly rounded tit 25 to bearing against the upper face of the spring.

Due to the interacting relation of expansible casing lo and bimetallic element 17 to perform two distinct useful functions in consecutive order, the over-all diameter of the switch as a whole has been extremely reduced in the interest of miniaturization. The actual diameter ot the smaller switches is seven thirty-seconds of an inch. Because of the even contact-wiping action under proper pressure throughout each such action, the life of the switch while permanently sealed has been greatly increased beyond that of any of the prior art devices of which l am aware.

lt will be understood that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purpose of illustration which do not constitute departures from the spirit of the invention and scope of the appended claims.

l claim:

l. A thermostatic switch comprising: an elongated, substantially straight terminally closed tubular, metallic `frame; a substantially straight bimetallic element in strip form arranged substantially parallel to the axis of said frame inside the same with its base end rigidly supported at one end of the frame and its opposite end free; and a coacting pair of electrical contacts of which one is stationarily supported by said frame in radially direct electrical connection therewith adjacent to the free end of the bimetallic element and the other is movable laterally with respect to said frame by temperature-responsive movement of the bimetallic element and is maintained in lateral registration with the rst-mentioned stationary Contact, said frame and at least one metallic component of the bimetallic element being possessed of different coeihcients of expansion whereby heat penetration from the frame to the bimetallic element is retarded so that axial wiping will occur between the meeting faces of both contacts due to initial linear expansion of said frame under the influence of changes in ambient temperature while said contacts are in closed condition.

2. A thermostatic switch as defined in claim l, wherein the movable contact is directly supported by the free end of the bimetallic element.

3. A thermostat-ic switch as defined in claim l, to which is 'added a substantially straight linear spring having `a coeicient of expansion differing from that or" the frame `and arranged substantially adjacently parallel to the bimetallic element with one end rigid with the rigidly supported end of the latter and with the opposite end `free and projecting beyond the free end of said element and being in electrical bearing contact therewith, the movable electrical contact being supported by the free end of said spring and in contact-closed condition being pressed against the stationary contact.

4. A thermostatic switch as defined in claim l, to which is added a radially extending screw radially adjustable in the tubular frame and supports the stationary contact at its inner end to permit adjustment of the latter toward and away from the coacting movable contact.

Donle Apr. 20, 1926 Fitzmaurice May 6, 1941. 

